FIG. 1 illustrates nine pixels of a Liquid-Crystal Display (LCD). Each box labeled DEVICE represents one of the LCD elements. Each LCD element is called a pixel. The transistors labeled MOS turn their respective pixels on and off.
The operation of the LCD can be explained, in a very simplified manner, as follows. In FIG. 2, liquid crystal material M is contained between the plates P of a capacitor C. (Each box labeled "DEVICE" in FIG. 1 contains one of the devices shown in FIG. 2.)
Each plate P in FIG. 2 actually takes the form of a thin coating of Indium Tin Oxide, ITO, on GLASS, as indicated in FIG. 3A. Each coating of ITO, in turn, bears a coating of polyimide, as indicated in the insert 4 shown in FIG. 3C. The polyimide has been rubbed, during manufacture, in a unidirectional manner. The rubbing causes the molecules of the liquid crystal material, which are adjacent to the polyimide, to align with the direction of rubbing. For example, molecules M1 and M2 align as shown.
The polyimide layers are arranged such that the aligned molecules M1 and M2 are perpendicular to each other, as shown. The molecules located in the bulk of the liquid crystal try to align themselves with M1 and M2, but, because M1 and M2 are perpendicular, the bulk molecules align into a helix H which bridges M1 and M2.
Polarizing filters are affixed to each sheet of GLASS, as indicated. When incoming LIGHT enters, as shown in FIG. 3B, the polarization of the LIGHT follows the twisted molecules, and the LIGHT undergoes a continuous 90-degree twist, as shown, and exits through the bottom polarizing filter. The human EYE, perceives the pixel as bright, because of the exiting LIGHT.
However, when a small voltage (such as 3-5 volts) is applied between the ITO plates, the voltage creates an ELECTRIC FIELD in FIG. 4, which disturbs the gradual twist of the molecules. The helix no longer exists. The light is no longer twisted as it travels, but is blocked by the lower polarizing filter, as shown in FIG. 5. The pixel appears dark.
In an actual LCD, the number of pixels is quite large. For example, the display of a small computer can contain an array of 480.times.640 pixels, giving a total of 307,200 pixels. With such a large number of pixels, the voltage described in connection with FIG. 4 is applied to each MOS in multiplex fashion.
In multiplexing, there exists an external Random-Access Memory (termed Video RAM, or V-RAM) which contains a memory cell for each pixel. A video controller (not shown) writes data, which represents the image to be displayed, into the V-RAM. Then, other circuitry (not shown) reads each cell in the V-RAM, and applies the proper voltage to the corresponding MOS, causing each pixel to become bright or dark, as appropriate.
The charge which produces the ELECTRIC FIELD shown in FIG. 4 does not last forever, but dissipates with time. Consequently, the video display is "refreshed" periodically, to restore the charge. In one type of refreshing, a controller (indicated in FIG. 1) reads each memory cell in V-RAM, and applies the proper charge to each MOS of each pixel, based on the memory cell's contents.
FIG. 1 is somewhat exaggerated for clarity: the Metal Oxide Semiconductor Transistors (MOS) actually occupy proportionately less space than shown, and the DISPLAY occupies greater space. The reason for giving the DISPLAY element more space is to allocate maximum area to the information-producing component, namely, the DISPLAY element.
LCD displays are becoming widely used, especially in portable computers having pen-type input devices (which augment, or substitute for, keyboard input). It is desirable to provide an improved system for responding to pen-type input.